Process for increasing the scaling resistance of titanium base metals



United States Patent PROCESS FOR INCREASING THE SCALING RE- SISTANCE 0F TITANIUM BASE METALS Karl Hautfe, Dusseldorf, Germany, assignor to Deutsche Goldand Silber-Scheideanstalt vormals Roessler, Frankfurt am Main, Germany No Drawing. Application February 2, 1954 Serial No. 407,800

Claims priority, application Germany February 3, 1953 4 Claims. (Cl. 148-6.3)

The present invention relates to a process for increasing the scaling resistance of metals and is based upon new concepts concerning the formation and structure of the oxide layers or coatings which are formed as scales or tarnishes upon the metals upon attack by oxygen especially at higher temperatures.

As is well known, such oxide layers or coatings only protect the metals in question against further attack by oxygen in a very limited number of instances. Normally either the oxygen dilfuses through the oxide layer and continues to react with the metal therebelow or the metal, in the form of ions and electrons wanders out through the oxide layer where it can, react with the oxygen, so that the oxide layer does not effectively prevent continued attack upon the metal base.

In accordance with the invention, it has been discovered that such ditfusion of ions and electrons through the crystal lattice of the oxide scale or tarnish layer is only possible if different positions in such lattice are not occupied or if additional ions can be built in between the actual lattice positions on the so-called interstitial lattice positions. In general the crystals of an oxide scale coating are not formed stoichiometrically and when unoccupied cation positions in the space lattice are present, an equivalent quantity of electrons are lacking because of the electron neutrality, so that to a certain degree, electron holes, hereafter termed electron defect positions, are present in the lattice. If, correspondingly, additional cations are located in interstitial lattice positions, an equivalent number of quasi-free electrons will be present in the lattice, and consequently, the lattice contains an excess of electrons. Consequently, the oxides with electron defect positions in the lattice are designated as electron-defect-conductors (p-conductors) and those containing excess quasi-free electrons in the lattice are designated as electron-excess-conductors (n-conductors). Practically only those ions or electrons are movable which are defectively positioned in the lattice.

In electron conducting oxide coatings formed on metals, the partial conductivity of the ions is low in comparison to that of the electrons so that the speed of oxidation of metals which are in the position to form such coatings, is determined by the partial conductivity of the ions, and that such velocity then assumes small values when a low concentration of defectively positioned ions is present.

It was found according to the invention that in electron conducting oxide coatings, that the number of ion defect positions and therefore the rate of scaling of the base metal in an oxygen containing atmosphere can be reduced when such metal ions are introduced in the oxide coatings formed, which increase the electrical conductivity of the oxide scale It is therefore possible to ascertain what ions are in a position to increase the scaling resistance of metals by simple routine tests.

Especially good results are obtained according to the invention, when the metal ion introduced into the oxide scale layer is selected with reference to whether such 2,842,470 Patented July 8, 1958 scale layer formed on the metal to be protectedis built up of oxides which are electron-defect-conductors or whether it is built up of oxides which are electron-excessconductors. It was found that, in. the first instance, namely, in electron-defect-conducting layers, the number of ion defect positions and therefore the velocity of scaling can be lowered by incorporating such cations which have a lower valence than the cations of the lattice of the oxide scale and that in the case of electronexcess-conducting layers this effect can be achieved with cations having a higher valence than the cations of the lattice of the oxide scale. For example, when nickel is to be protected against scaling, a compound of a metal of lower valence than nickel, for example, a lithium compound such as lithium oxide, would, in accordance with the invention, be incorporated in the nickel oxide layer. On the other hand, in order to increase the resistance to scaling of zinc, which forms an electron-excess-conducting oxide, a cation of a metal having a higher valence than zinc, for example, aluminum would be employed.

The process according to the invention has been found particularly suited for treatment of metals of the iron group and their alloys, especially nickel and nickel 'rich alloys such as nickel chromium alloys. The process also produces good results with other metals and especially in protecting titanium and its alloys against scaling.

The metal ions which are incorporated in oxide scale coatings according to the invention to increase the scaling resistance of the metal base are preferably incorporated as oxides by difiusion into the oxide scale as it is being formed or after it has already been formed on the metal base. Such diffusion can be accomplished in the gas phase, that is by subjecting the scale being formed or after it has been formed to a vaporized oxide at a temperature at which it diffuses into the scale or by direct contact with powdered oxide packing compositions as in pack carburization, and heating to diffusing temperatures. It is furthermore possible to incorporate the desired metal ion in the scale as it is formed, in certain instances and especially in the case of titanium base metals by alloying the metal which is capable of forming an oxide capable of increasing the electrical conductivity of the scale formed directly in the base metal. It is also possible to use a combination of the above methods, for example, by alloying the desired metal in the base metal and then packing such alloyed base metal in a packing composition containing the oxide of such metal and heating the packed alloyed base metal with access to air until the desired scale, having the metal oxide increasing its electrical conductivity incorporated therein, is produced.

When oxide mixtures are employed as packing compositions which, for example, essentially can consist of inert high melting oxides such as aluminum or silicon oxides and relatively smaller amounts of the oxide desired to be incorporated in the oxide scale, care must be taken that the quantity of the oxide to be incorporated in the mixture is sufiicient that such oxide is present in a heterogeneous phase at the difliusion temperature employed. In other words, the quantity of the oxide to be incorporated contained in the oxide mixture must be above the quantity which would be dissolved in the inert diluent oxide at the temperatures employed for the diffusion treatment.

The process according to the invention can beemployed to increase the scaling resistance of metals or metal alloys which are already rather scaling resistant, but the process is, of course, of special advantage in increasing the corrosion resistances of metals or metal alloys which are to be employed under such conditions where their resistance to scaling normally would not suffice. For example, nickel or nickel rich alloys can be treated by the process according to the invention in order to render them especially resistant to scaling when employed in chemical apparatus or as heating conductors in furnaces.

An especial advantage of the process according to the invention is that it practically does not alter the mechanical technical properties or the physical properties of the base metal treated. The quantity of the foreign oxide incorporated in the oxide scale coating to increase the electrical conductivity of such coating can amount to up to mol percent and preferably amounts only up to 2 mol percent.

When nickel or nickel alloys are to be rendered scaling resistant for use as electric heating elements, it is not, for example, necessary first to produce a nickel oxide coating on such elements by treatment in an oxidizing atmosphere and then to diffuse the desired oxide to increase the electrical conductivity of such nickel oxide coating. It, to the contrary, is rather advantageous merely to pack the heating elements in a packing composition composed of, for example, a mixture of powdered alumina containing 0.05 to 5 mol percent of lithium oxide and heat the packed elements with access to an oxygen containing atmosphere, for example, air, to temperatures between 1000ll00 C. for a number of hours until the required amount of lithium oxide has been incorporated in the nickel oxide coating formed during such treatment.

The process according to the invention is also well suited for increasing the scaling resistance of titanium or titanium rich alloys. This is of special importance as titanium, because of its low density, is especially suited for use in blades of gas turbines. Nevertheless, the scaling resistance of titanium at temperatures above 600 C. is normally rather poor.

It was found, according to the invention, that oxides of metals of higher valence than titanium, such as tungsten, molybdenum and vanadium oxide, are well suited for the treatment of titanium or titanium rich alloys to increase its resistance against oxidation. The titanium articles to be treated can, for example, be packed in an oxide mixture which besides alumina and titanium dioxide, contain 0.05 to 3 mol percent of tungsten trioxide and then heated under access of oxygen to temperatures between 800 to 1100 C., preferably 1000 to 1050 C. until the desired titanium oxide coating having tungsten oxide incorporated therein is formed. It is of no consequence if the originally loose packing material sinters together to form a rather compact mass during the treatment.

The following examples will serve to illustrate the manner in which the process according to the invention can be carried out.

Example 1 Iron articles were packed in a loose chromium oxide powder and then heated for 4 hours at 1000 C. with access to air. The scale layer formed had chromium oxide incorporated therein which increased the electrical conductivity thereof and rendered the treated articles considerably more scaling resistant. Equally good scaling resistance was obtained when the chromium oxide powder was replaced by aluminum oxide.

Example 2 Iron articles were packed in a loose powder mixture of chromium oxideand aluminum oxide (1:1) containing 0.5 mol percent of beryllium oxide and heated for 4 hours at 1000 C. with access to air. A very uniform firmly adherent oxide scale coating, which protected the iron against further scaling was obtained thereby. The function of the beryllium oxide in the packing composition was to render the scale produced more firmly adherent to the iron base. Similar results were obtained when the beryllium oxide was replaced by oxides of thorium, cerium or calcium or mixtures of such oxides.

4 Example 3 Iron articles were packed in a loose powder mixture composed of 50 mol percent of chromium oxide, 40 mol percent of nickel oxide, 9.5 mol percent of aluminum oxide and 0.5 mol percent of thorium oxide and heated with access to air for 4 hours at 900 C. The oxide scale layer formed provided good protection against fur ther scaling.

Example 4 An ordinary low alloy ferritic carbon steel containing about 0.2% carbon and the-usual alloying elements was treated in the same manner as described in Examples 1 through 3. The resulting oxide scale in each instance provided good protection against further scaling.

Example 5 A ferritic steel of the following composition: 0.l5% C, 0.5% Si, 0.10.6% Mn and 1-3% Cr was treated in the same manner as described in Examples 1-3. The resulting oxide scale in each instance provided excellent protection against scaling and the steel articles thus treated had a scaling resistance similar to that of a high chromium austenitic steels.

Example 6 Nickel wires were packed in a powdered mixture of aluminum oxide and 0.5 mol percent of lithium oxide and were heated therein with access to air for 4 hours at 1050 C. The resulting oxide scale on the nickel wires provided good protection against further scaling. Similar results were obtained when nickel-chromium wires containing 16% chromium were treated in the same packing composition at the same temperature for 5 hours.

Example 7 Shaped titanium bodies were packed in a powder composed of tungsten oxide containing 0.5% of thorium oxide and were heated therein with access to air to 900 C. for 5 hours. The resulting oxide scale provided good protection against further scaling. The thorium oxide served to increase the bond of the scale to the base metal. Similar results were obtained when the thorium oxide was partially or completely replaced by one or more of the oxides of cerium, beryllium or calcium.

Example 8 A shaped titanium alloy article composed of titanium, 5 atom percent of tungsten and 0.5 atom percent of thorium was heated in air for 4 /2 hours at 1000 C. to produce an oxide scale thereon. The resulting scale provided good protection against further scaling.

Example 9 A shaped body of the same alloy as in Example 8 was packed in a powder mixture of the same composition as that employed in Example 7 and heated therein with access to air at 950 C. for 5 hours. The resulting finely adherent oxide scale coating again provided good protection against further scaling.

Example 10 A shaped body of titanium was packed in a powdered mixture essentially composed of aluminum oxide, titanium dioxide and 3 mol percent of tungsten trioxide and heated therein with access to air for 4 /2 hours at 1000 C. The resulting oxide scale provided good protection against further scaling. Similar results were obtained in treating titanium alloys containing molybdenum or vanadium when the tungsten oxide of the packing composition was replaced by molybdenum or vanadium oxides.

I claim:

1. A process for increasing the scaling resistance of a titanium base metal which comprises forming a titanium oxide scale containing tungsten tn'oxide on said titanium base metal.

2. A process for increasing the scaling resistance of a titanium base metal which comprises packing such titanium base metal in a powdered packing composition containing tungsten trioxide and heating the packed titanium base metal to a temperature between 800 and 1100" C. in an oxidizing atmosphere until an oxide scale is formed on the titanium base metal which protects such titanium base metal against further scaling.

3. A process for increasing the scaling resistance of a titanium base metal which comprises packing such titanium base metal in a powdered packing composition containing tungsten trioxide and about 0.5 mol percent of an oxide selected from the group consisting of beryllium, thorium, cerium and calcium oxides and heating the packed titanium base metal to a temperature between 800 and 1100 C. in an oxidizing atmosphere until an oxide scale is formed on the titanium base metal which protects such titanium base metal against further scaling.

4. A process for increasing the scaling resistance of titanium which comprises alloying 0.05 to 5 atom percent of tungsten and about 0.5 atom percent of a metal selected from the group consisting of beryllium, cerium, thorium and calcium in said titanium and then heating such alloyed titanium to a temperature of between 800 and 1100 C. in an oxidizing atmosphere until an oxide scale is formed which protects such titanium alloy against further scaling.

References Cited in the file of this patent UNITED STATES PATENTS 1,008,807 Farkas Nov. 14, 1911 1,813,320 Schaefer et a1. July 7, 1931 1,926,407 Ruben Sept. 12, 1933 2,162,362 Smith June 13, 1939 2,263,366 Peck et a1. Nov. 18, 1941 2,269,601 Perrin Ian. 13, 1942 2,465,105 Levi Mar. 22, 1949 FOREIGN PATENTS 462,380 Great Britain Mar. 4, 1937 513,276 Great Britain Oct. 9, 1939 

4. A PROCESS FOR INCREASING THE SCALING RESISTANCE OF TITANIUM WHICH COMPRISES ALLOYING 0.05 TO 5 ATOM PERCENT OF TUNGSTEN AND ABOUT 0.5 ATOM PERCENT OF A METAL SELECTED FROM THE GROUP CONSISTING OF BERYLLIUM, CERIUM, THORIUM AND CALCIUM IN SAID TITANIUM AND THEN HEATING SUCH ALLOYED TITANIUM TO A TEMPERATURE OF BETWEEN 800 AND 1100*C. IN AN OXIDIZING ATMOSPHERE UNTIL AN OXIDE SCALE IS FORMED WHICH PROTECTS SUCH TITANIUM ALLOY AGAINST FURTHER SCALING. 